How and Why to Carb Load? A Guide for Endurance Athletes

October 18, 2025 •

5 min read

A proper carb loading strategy can boost athletic performance by 2-3% and enhance endurance by up to 25% during events lasting longer than 90 minutes. This tactical approach to nutrition is crucial for endurance athletes seeking to maximize their energy reserves and avoid premature fatigue.

Quick Summary

This guide explains the science behind carbohydrate loading, a strategy used to maximize muscle and liver glycogen stores before a prolonged endurance event. It details modern loading protocols, proper dietary adjustments, common mistakes to avoid, and offers a sample meal plan to help athletes execute the strategy effectively.

Key Points

Timing is Key: Carb load over 1–3 days, not just the night before, for optimal glycogen saturation.
Calculate Needs: Target 8–12 grams of carbohydrates per kilogram of body weight, adjusted for individual tolerance.
Adjust Macronutrients: Reduce fat and fiber intake during loading to prioritize carbs and prevent gastrointestinal issues.
Practice During Training: Never try a new carb-loading plan or unfamiliar foods on race week to avoid unexpected digestive problems.
Embrace Water Weight: Temporary weight gain is normal and beneficial, as glycogen binds with water, aiding hydration during your event.
Combine with Tapering: Reduce your training volume during the loading phase to ensure your body stores, rather than burns, the extra carbohydrates.
Not for Short Events: Carb loading is most effective for events lasting over 90 minutes and is not necessary for shorter, less intense exercise.

Carbohydrate loading, often simply called 'carb loading,' is a nutritional strategy used by endurance athletes to increase the storage of glycogen in their muscles and liver before competition. Glycogen is the stored form of glucose, and it serves as the body's primary and most readily available fuel source, particularly during high-intensity exercise. While the image of an athlete eating a massive bowl of pasta the night before a race is common, the modern approach is more nuanced, science-backed, and effective.

The 'Why': The Science Behind Carb Loading

For any high-intensity activity, your body relies on glycogen for energy. However, the body can only store a finite amount of this fuel. During prolonged endurance exercise, such as a marathon or triathlon, these glycogen stores can become depleted after approximately 90 minutes. When this happens, an athlete may experience a sudden, overwhelming sense of fatigue and a sharp drop in performance, a phenomenon commonly known as "hitting the wall".

Carb loading strategically increases your muscle glycogen reserves above their normal capacity. By entering an event with a fully topped-up fuel tank, athletes can prolong their energy supply, delay the onset of fatigue, and maintain a higher intensity level for longer. Research has shown that effective carb loading can significantly enhance endurance and improve time trial performance in events over 60-90 minutes. It essentially buys you more time and power when you need it most.

The Physiological Mechanism

Enhanced Glycogen Storage: By temporarily increasing carbohydrate intake while decreasing exercise volume, your body becomes more efficient at storing glycogen. The combination of reduced energy expenditure and increased fuel availability signals your body to maximize its reserves.
Water Retention: One gram of stored glycogen holds roughly three grams of water. This is why athletes may experience temporary weight gain during a carb load. This water retention is beneficial, as it can help delay dehydration during the race.
Prevents Hypoglycemia: Adequate glycogen stores, particularly in the liver, are crucial for maintaining stable blood sugar levels during prolonged exercise, preventing the symptoms of low blood sugar that can impair performance.

The 'How': Executing a Modern Carb-Loading Protocol

Modern carb-loading strategies are more straightforward than the outdated depletion-phase methods. The goal is to consistently increase carbohydrate intake for 1-3 days leading up to the event, rather than depleting your body first.

Timing is Crucial: Begin your carb load 36 to 48 hours before the event. A Sunday race, for example, means starting Friday morning or evening. This provides ample time to top off glycogen stores without causing discomfort.
Calculate Your Intake: Aim for 8–12 grams of carbohydrates per kilogram of body weight per day. For a 70kg athlete, this equates to 560–840 grams of carbs daily. For many, this is a significant increase and requires intentional planning.
Prioritize Carbohydrates: To achieve this high intake without overeating, you must reduce your consumption of fat and fiber. Fatty foods add excess calories that can leave you feeling sluggish, while high-fiber foods can cause gastrointestinal distress on race day.
Drink Your Carbs: If eating enough carbohydrates is a challenge, incorporate high-carb fluids like fruit juice, sports drinks, and smoothies. This is an efficient way to increase your intake without feeling overly full.
Stay Hydrated: Since glycogen binds to water, staying hydrated is essential. Drink plenty of water and other fluids during the loading process.
Taper Your Training: Reduce the volume and intensity of your exercise in the days leading up to your race. This allows your muscles to store the increased carbohydrate intake rather than burning it off.

Carb Loading: A Strategic Comparison


Aspect	Proper Carb Loading	The Pasta Party Myth	General High Carb Diet
Timing	1–3 days before the event	The night before the event	Daily, for general health
Food Choices	Easily digestible, low-fiber carbs (e.g., white pasta, white rice, juice)	Often high-fiber and high-fat (creamy sauces, large portions of whole grains)	Emphasis on complex carbs and fiber
Training Changes	Reduced training volume (tapering)	None considered	Varied based on training demands
Key Outcome	Maximized glycogen stores for sustained energy	Risk of stomach upset and incomplete glycogen loading	Replenishes daily glycogen, but not supercompensated for an event
Drawbacks	Temporary water weight gain, potential minor bloating	Potential GI distress, feeling sluggish, insufficient fueling	Less effective for maximizing performance during prolonged events

Common Carb-Loading Mistakes to Avoid

Loading for the Wrong Event: Carb loading is not necessary for exercise lasting less than 90 minutes. For a 5k or an hour-long gym session, your regular daily diet is sufficient.
Skipping the Taper: If you continue to train hard during the loading phase, you'll simply burn off the extra carbs, defeating the purpose of the strategy.
Ignoring Fiber: While fiber is healthy, it can cause digestive problems when consumed in large quantities right before an event. Switch to low-fiber options like white bread and rice.
Eating Too Much Fat: Excessive fat intake displaces carbohydrate grams and can leave you feeling full and sluggish. Opt for light sauces and lean protein sources.
Trying New Foods: Race week is not the time to experiment. Stick to familiar foods that you know your body tolerates well to avoid unwelcome digestive surprises.

Conclusion: Fueling Your Peak Performance

Executing a proper carb-loading strategy is a science-backed method for endurance athletes looking to gain a competitive edge in events over 90 minutes. It moves beyond the simplistic 'pasta party' of old, requiring careful planning over several days to maximize your body's energy reserves. By focusing on low-fiber, high-glycemic carbohydrates, reducing training, and staying hydrated, you can effectively top off your fuel tank. This careful preparation is the final, crucial step in ensuring your body is primed for peak performance when it matters most, allowing you to run, cycle, or swim further and faster before the onset of fatigue. For further guidance and individualized plans, consider consulting a sports dietitian.

Example Carb Loading Day (for a 70kg athlete aiming for 700g carbs/day):

Breakfast (approx. 150g carbs): Large bowl of oatmeal made with milk and honey, a large banana, and a glass of orange juice.
Morning Snack (approx. 75g carbs): Two plain bagels with jam and a small energy bar.
Lunch (approx. 175g carbs): Two large cups of white rice with a lean protein source like chicken breast (small portion) and a glass of apple juice.
Afternoon Snack (approx. 100g carbs): A large portion of pretzels and dried fruit.
Dinner (approx. 150g carbs): Large serving of white pasta with a simple marinara sauce, a side of garlic bread, and a sports drink.
Evening Snack (approx. 50g carbs): Low-fat yogurt with a few scoops of honey.

Frequently Asked Questions

The primary purpose of carb loading is to maximize muscle and liver glycogen stores, which serve as the main energy source for high-intensity exercise. This helps to delay the onset of fatigue and sustain performance during endurance events lasting longer than 90 minutes.

Carb loading is beneficial for endurance athletes preparing for events like marathons, triathlons, or long-distance cycling. It is not necessary for shorter duration activities (under 90 minutes) or for general fitness enthusiasts, and may lead to unwanted weight gain and discomfort.

Modern carb-loading protocols recommend increasing carbohydrate intake for 1 to 3 days (36-48 hours) before an event. The older, more rigorous 'depletion' phase is no longer considered necessary for most athletes.

Yes, you will likely experience a temporary weight gain of 2-4 pounds during carb loading. This is primarily water weight, as each gram of stored glycogen binds with three grams of water, which is a beneficial sign that your fueling is on track.

No, it is recommended to reduce fiber intake during carb loading to avoid gastrointestinal issues like bloating and discomfort on race day. Stick to easily digestible, low-fiber sources such as white rice, white bread, and refined pasta.

Relying on one large meal the night before can cause stomach discomfort and is not as effective as consistently increasing your carb intake over several days. A prolonged loading phase ensures your glycogen stores are fully topped off without stressing your digestive system.

Reducing your training volume, a process known as tapering, is essential during carb loading. It ensures that the extra carbohydrates you consume are stored as glycogen instead of being burned during exercise, thereby maximizing your reserves for the race.